1. Field of the Invention
The present invention relates to an air bag, which is used in an air bag device to located in vehicles to protect the driver and/or passengers in the event of a collision. An air bag for vehicles (hereinafter simply referred to as an "air bag") is instantaneously inflated, when triggered for operation, by quick injection of an expansive gas from an inflator, thus protecting the driver and/or passengers.
2. Description of the Prior Art
A conventional air bag 1 is shown in FIGS. 1 to 3.
Abbreviations for polymers and plasticizers referred to hereinafter are as follows:
CR : Chloroprene Rubber PA1 IIR : Butyl Rubber PA1 EPR : Ethylene Propylene Rubber PA1 EPDM : Ethylene Propylene Non-conjugated Diene Terpolymers PA1 EPM : Ethylene Propylene Copolymer PA1 DOS : Di(2-ethylhexyl) Sebacate
Unless otherwise specifically indicated, compounding unit is weight.
The air bag 1 is in a bag shape. It is formed by sewing together, with a sewing thread 6, the peripheral portions of two sheets 4 and 5 of composite materials, as shown in FIG. 1. Each of the sheets 4 and 5 is composed of a base fabric 2 having laminated on one side thereof a heat-resistant film 3 made of a CR compound. When the two sheets 4 and 5 are sewed together into a bag, the peripheral portions of the sheets and the heat-resistant film 3 are located inside the bag with the fabric on the outside.
The outer peripheral portion of the film 3 has a protective layer 8 thereon to cover the sewed areas of the sheets 4 and 5, i.e. to cover the exposed portions of the thread 6 and protect them from the heat of an expansive gas which has high temperature initially when it is injected into the air bag 1.
That sheet of the air bag 1, e.g. sheet 4, opposite the sheet facing a person is provided with a central inflator-mounting hole 9. A reinforcing composite substrate 12, unually annular, is sewed inside the bag on the sheet 4 around the inflator-mounting hole 9. the substrate 12 has the same composition as that of the sheets 4 and 5, i.e. having a reinforcing base fabric 10 having laminated on one side thereof a heat-resistant film 11 made of a CR compound. the film 11 faces the interior of the bag 1. The inside area 13 where the substrate 12 and the sheet 4 are sewed together is covered with a protecting layer 14, like layer 8, to protect the exposed portions of the sewing thread 6 attaching the substrate 12 to the sheet 4, from the heat of the expansive gas.
Additionally, reinforcing composite substrates 12 inside the bag 1 are sewed onto the sheet 4 about the periphery of gas exhaust holes 15 in the sheet 4. Those sewed areas are also covered with layers 17 to protect the threads from expansive gas heat.
Through holes 18 are provided in the sheet 4, substrate 12 and layer 14 for bolts and rivets (not shown) to mount the inflator I on the sheet 4.
Conventionally, the sewing thread protecting layers 8, 14 and 17 are generally made of a silicone rubber compound (Refer to Japanese Utility Model Laid-Open Publication No. Sho. 61-85549). Specifically, a silicone rubber tape or silicone rubber glue is applied on the sewed areas to form the thread-protecting layers. A silicone rubber compound is used to make the layers is because it is excellent in both heat resistance and cold-crack resistance.
However, a thread-protecting layer of a silicone rubber compound does not have good adhesion with a heat-resistant film made of a CR rubber compound.
A suggestion may be made to form the thread-protecting layer with a CR rubber compound similar to that of the heat-resistant film, but that would result in problems.
In the first place, it is necessary to make the protecting layer thicker than the heat-resistant film because the protecting layer must not only shield the sewing thread but also smooth out the rough surface of the heat-resistant film caused by the sewing thread. Thus, the thickness of the protecting layer is gernerally from 0.2 to 2 mm and the heat-resistant layer from 0.02 to 0.15 mm.
However, when the thicker protecting layer is formed with a CR compound, its inferior cold-crack resistance performance versus that of silicone rubber is immediately evident.
Hence, it has been known to those skilled in the art that CR is undesirable as a material for the thread-protecting layer.
For installing the air bag 1 in a car, it is folded into a small and compact form and then installed in an air bag device. When folded into a small and compact form, the peripheral por of the sheets 4 and 5 are folded many more times than the areas of sheet 4 in proximity to the inflator-mounting hole 9 and the areas of the sheet 5 opposite that hole. Therefore, the material chosen for the protecting layer 8 on peripheral portions of the sheets 4 and 5 is influenced by the cold-crack resistance factor.